Many human cancers require the production of soluble growth factors for tumor initiation, promotion and survival. These extracellular factors contribute to and promote most stages of tumor development. For example, tumors of the mammary gland are strongly influenced by the activity of epidermal growth factor (EGF) receptor family members. EGF binding to its cognate receptor is thought to contribute to breast cancer cell cycle regulation by activating signaling pathways that facilitate cyclin D1 expression, cyclin- dependent kinase 4 or 6 (Cdk4t6) activation and, ultimately, retinoblastoma (Rb) inactivation. While Rb inactivation is critically important for oncogene-induced malignancies, the nature of the signals induced by oncogenes to facilitate this are incompletely understood. We recently discovered that the pro-inflammatory cytokine, migration inhibitory factor (MIF) is both necessary and sufficient for mitogen and oncogene-induced cyclin D1 transcription, Cdk4 activity and Rb inactivation. Moreover, our results reveal that MIF is strongly induced by tumor promoting oncogenes and cells from MIF-deficient mice are resistant to oncogene-induced malignant transformation. Despite these findings, more work is needed to investigate in detail the influence, mechanism and effectors of MIFs contribution to Rb inactivation, malignant growth properties and de novo tumorigenesis. We hypothesize that MIF promotes both normal and neoplastic cell growth by stimulating RhoA GTPase activity that leads to the activation of the canonical MAP kinase pathway and resulting in cyclin D1 transcription and Rb inactivation. To test the fundamentals of our hypothesis and fulfill the stated objectives of this application, the following specific aims are proposed: 1) Examine the regulatory and effector requirements for MIF in cyclin D1 transcription focusing on Rho GTPase activated pathways; 2) Test the requirements for MIF in human breast carcinoma Rho activation, cyclin D1 expression and Rb inactivation, and; 3) Investigate the contribution and functional requirements for MIF in de novo mammary tumorigenesis. This work should contribute to a greater understanding of the physiologic and pathologic importance of soluble growth factors to cell cycle regulation and neoplastic processes and may reveal a novel target for future cancer therapies.